Co-existing static and dynamic IP multicast

ABSTRACT

A system and method are provided for providing both static and dynamic IP multicasting. The concept of a multicast Static-Range is introduced which allows the coexistence of static and dynamic IP multicast. The multicast Static-Range is a set of Class D IP addresses which is reserved for static multicasting, and is configured at all routers. When a router receives a PIM message or an IGMP message, the router determines whether the group specified in the message is within the multicast Static-Range. If the group pertains to a static multicasting group, the router does not propagate the message to upstream routers using PIM-SM or PIM-SSM protocols, and only connects or disconnects interfaces internal to the router. If the multicast group address in the message is not within the multicast Static-Range, the router recognizes that the message pertains to a dynamic multicasting group and implements PIM or IGMP protocols as usual. If the invention is used for broadcasting TV, the low end of TB channels or commonly used channels can be created as static IP multicast. This way, a user can access or leave such channels without an entire shortest path tree being created or torn down, improving access time and channel surfing for a user. Also, pay-per-view, digital or less frequently used channels can be created using dynamic IP multicast with traditional PIM-SSM protocol, in order to make efficient use of router resources.

FIELD OF THE INVENTION

The invention relates to IP multicasting in communication networks, andmore particularly to IP address ranges for use in creation of IPmulticast distribution trees.

BACKGROUND OF THE INVENTION

To establish a multicast tree from a source which provides multicastcontent to several hosts which desire to receive the traffic, twogeneral approaches are Dynamic IP Multicast and Static IP Multicast. Indynamic multicast, a multicast protocol such as Protocol IndependentMulticast-Sparse Mode (PIM-SM) or Protocol Independent Multicast-SourceSpecific Mode (PIM-SMM) is used to create and propagate the multicasttree from multicast hosts to a multicast source. In static multicast,the multicast protocol is not responsible for establishing andpropagating the multicast tree. Rather, establishment of the multicasttree is effected by other means such as network management or throughconfiguration.

In PIM-SM, a shortest path tree (SPT) can not be established directlybetween the source and hosts because the hosts are unaware of theidentification of the source. The hosts can only identify the content asbelonging to a multicast group. Rendezvous Points (RPs) are routers usedas temporary waypoints in order to assist in creation of the SPT. AUrouters have access to an identical mapping between multicast groups andRPs, one RP being associated with each group. When a router receives anIGMP-JOIN(*,G) or a PIM-JOIN(*,G) message, the router establishes aroute towards the RP associated with the group. As numerous routersestablish routes to the RP, an RP tree (RPT) is established. The routerconnected to the source (the source router) has the same mapping betweenRPs and groups. When the source begins transmitting IP packets for agroup, the source router starts registration of the packets with the RPby encapsulating the IP packets and forwarding them to the RP. As therouters within the RPT connected to the hosts receive the IP packets viathe RPT, they learn the identification of the source, create SPT routesto the source, and tear down the routes to the RP. The RTP is therebyconverted into a SPT.

In PIM-SMM, the hosts are aware of the source of the multicast contentfor a given group. In an example of PIM-SMM being used for broadcastingTB channels, when a user selects a channel to watch, a set top box (STB)determines the multicast group G and a source S for the channel andsends an IGMP JOIN(S,G) message to a host router. The multicast group isdefined by a Class D IP address within the range 224/4. The host routerfirst establishes internal connections between an outgoing interface(OIF) to the STB and an incoming interface (IIF) to the next router. Thehost router then determines the next router through which the source canbe reached, and forwards a PIM JOIN(S,G) message to the next upstreamrouter. The next router creates its own internal connections andforwards the PIM JOIN(S,G) message to an upstream router towards thesource S. This is repeated until a PIM JOIN(S,G) message reaches eitherthe router attached to the source or reaches a router which already has(S,G) state.

If any router receives a PIM JOIN(S,G) message and the multicastdistribution tree is already supported for (S,G) at the router because aPIM JOIN(S,G) or an IGMP JOIN(S,G) for the same source and group havealready been received from a different router or another host, then therouter simply establishes the internal connection between the OIFthrough which the newly arrived PIM JOIN(S,G) message arrived and theIIF which leads to the source.

In PIM-SM, if a host no longer wishes access to the group, the hostsends an IGMP LEAVE(*,G) message to the host router. The host routerfirst removes the connection between the OIF of the host and the IIFleading to the RP. If no other hosts are currently willing to receivetraffic for the same group, then the host router then sends a PIMPRUNE(*,G) message to the upstream router towards the RP. This isrepeated by the upstream routers until either the RP or a routersupporting more than one host for the group is reached, at which pointthe internal connection to the OIF is removed and the PIM PRUNE(*,G)message is not forwarded.

In PIM-SMM, if a host no longer wishes access to the group, the hostsends an IGMP LEAVE(S,G) message to the host router. The host routerfirst removes the connection between the OIF of the host and the IIFleading to the next upstream router. If no other hosts are currentlywilling to receive traffic for the same group and from the same sourcethen the host router then sends a PIM PRUNE(S,G) message to the upstreamrouter. This is repeated by the upstream routers until either the sourcerouter or a router supporting more than one host for the group andsource is reached, at which point the internal connection to the OIF isremoved and the PIM PRUNE(S,G) message is not forwarded.

In both flavors of PIM protocol, a shortest path tree between multiplehosts and a source is eventually generated and maintained as hostsrequest access to or leave the group and the source. Dynamic multicastresults in efficient use of interfaces, since each IIF in each router isgenerally shared by each downstream host, and each OIF in each router isgenerally shared by each downstream host of a downstream router.However, in applications in which hosts leave and join groupsfrequently, users may experience undesirable delay as RPTs and SPTs areestablished, torn down, or altered. An example of such an application isTV broadcasting.

In TV broadcasting applications of IP multicasting, users may frequentlyswitch the channel being watched as they scan channels for content orswitch between channels during commercials. When a user switches betweena first channel and a second channel, the STB first sends an IGMPLEAVE(S₁,G₁) message to the host router which may then send a PIMPRUNE(S₁,G₁) message upstream. The STB then sends an IGMP JOIN(S₂,G₂)message to the host router which may then send a PIM JOIN(S₂,G₂) messageupstream. Each time a user switches channels, a portion of the shortestpath tree is torn down and a new shortest path tree or portion of ashortest path tree is created. This consumes processing power at therouters, and more importantly (for the customer) can result inunacceptable delay in channel surfing because the PIM-SMM messages aretransmitted around the network to tear down the old multicast tree andrebuild a new one.

Static multicasting provides a solution to this problem. The multicastdistribution tree is established once for each multicast group (orchannel) from end-user hosts to multicast source without using anymulticast protocol. To setup a static multicast tree from a host forgroup G to a multicast source S, the multicast connections should becreated on each router from the host router to the multicast source,similar to the PIM-SMM protocol. However, static multicast lacks theflexibility offered by dynamic multicast in automatic creation andmaintenance of SPT trees as groups are added. Static multicast alsorequires that OIFs and IIFs be reserved for SPT trees, even if there areno hosts currently receiving multicast content over the SPT tree.

A system which allowed the coexistence of static multicast and dynamicmulticast without conflict would allow the advantages of each multicastmethod to be realized. Such a system must allow PIM-SM and PIM-SSMprotocols to run in a network in which static multicast states areprovisioned, without resulting in tearing down or corruption of thestatic multicast SPTs. Such a system should allow addition or removal ofdynamic multicast states and static multicast states independently ofeach other. In this way, more commonly used groups may be establishedusing static multicast and paths of less commonly used groups may beaccessed only as needed using the dynamic multicast. This would provideless delay to users as they switch between commonly used groups whilemaintaining efficient use of router resources.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method is provided forimplementing IP multicasting in a communication network. A multicastStatic-Range is defined, being a set of at least one Class D IP address.Each IP address within the multicast Static-Range is reserved for staticIP multicasting. At least one dynamic IP multicasting group isestablished in the network, each such dynamic IP multicasting grouphaving a respective IP address lying outside the multicast Static-Range.At least one static IP multicasting group is established in the network,each such static IP multicasting group having a respective IP addresslying within the multicast Static-Range, and PIM-SMM messaging andPIM-SM messaging is ignored for such groups.

In accordance with another aspect of the invention, a method is providedfor supporting coexistence of static IP multicasting and dynamic IPmulticasting at a router in a communication network. A multicastStatic-Range is stored in memory, the multicast Static-Range being a setof at least one Class D IP address reserved for static IP multicasting.Upon receipt of a PIM message or an IGMP message, it is determinedwhether the message pertains to a static multicasting group. If themessage does not pertain to a static multicasting group, the message isprocessed in accordance with dynamic IP multicast. If the message doespertain to a static multicasting group and the message is a PIM message,the message is discarded. If the message pertains to a staticmulticasting group and the message is an IGMP JOIN message specifying asource, a connection identifiable by the source and the group isestablished within the router without forwarding a PIM message to anupstream router. If the message pertains to a static multicasting groupand the message is an IGMP LEAVE message specifying a source, aconnection within the router and identifiable by the source and thegroup is removed, without forwarding a PIM message to an upstreamrouter.

In accordance with another aspect of the invention, a router is providedfor use in a communication network. The router includes a memory forstoring a multicast Static-Range, the multicast Static-Range being a setof at least one Class D IP address, each IP address within the multicastStatic-Range being reserved for static multicasting. The router includesmeans for determining, upon receipt of a message, whether the messagepertains to a static multicast group. The router includes means forignoring a received PIM message in the event that the message pertainsto a static multicast group. The router includes means for processing areceived PIM message in accordance with PIM protocol in the event thatthe message does not pertain to a static multicast group. The routerincludes means for, upon receipt of an IGMP message, determining whetherthe message pertains to a static multicast group. The router includesmeans for, upon receipt of an IGMP JOIN message specifying a source,establishing a connection within the router without forwarding acorresponding PIM message to an upstream router in the event that theIGMP JOIN message pertains to a static multicast group. The routerincludes means for, upon receipt of an IGMP LEAVE message specifying asource, removing a connection within the router without forwarding acorresponding PIM message to an upstream router in the event that theIGMP LEAVE message pertains to a static multicast group. The routerincludes means for processing a received IGMP message in accordance withdynamic IP multicast in the event that the message does not pertain to astatic multicast group.

The methods of the invention may be stored on computer-readable media asinstructions executable by a processor.

The methods and apparatus of the present invention allow both dynamicand static IP multicast to coexist and allow the states created by bothmulticast methods to be maintained independently. By providing a rangeof Class D IP addresses which is defined and reserved for static IPmulticast, network routers can distinguish static multicast groups fromdynamic multicast groups and can apply a new rule set to PIM JOIN/PRUNEand IGMP JOIN/LEAVE messages. Static channels can thereby be added orremoved at the routers without triggering any dynamic multicast. Themethods and the apparatus of the invention may find particular use in TVbroadcasting, where commonly accessed TV channels may be configuredusing static multicast while less commonly accessed TV channels orspecialty channels (such as pay-per-view channels) may be configuredusing dynamic multicast.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention will become more apparentfrom the following detailed description of the preferred embodiment(s)with reference to the attached figures, wherein:

FIG. 1 is a block diagram of a portion of an example communicationnetwork;

FIG. 2 is a diagram of Class D IP address allocation according to oneembodiment of the invention;

FIG. 3 is a flowchart of a router of FIG. 1 processes PIM and IGMPmessages according to one embodiment of the invention;

FIG. 4 is a block diagram of the network of FIG. 1 in which an examplestatic IP multicasting group is configured according to one embodimentof the invention; and

FIG. 5 is a block diagram of the network of FIG. 1 in which a secondexample static IP multicasting group is configured according to oneembodiment of the invention.

It will be noted that in the attached figures, like features bearsimilar labels.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 1, a block diagram of a portion of an examplecommunication network is shown. A host 10, operated by a user, isconnected to the network through a host router 12. The host 10 may beany device capable of requesting and receiving IP multicast traffic,such as a set top box (STB). A multicast source 14 is connected to thenetwork through a source router 16, and offers IP multicast content formulticast group G₁. The host router 12 and the source router 16communicate through an intermediate router 18. The layout of FIG. 1 isfor example purposes only, and more generally there may be a pluralityof hosts, a plurality of host routers, a plurality of sources, aplurality of source routers, and a plurality of intermediate routersbetween host routers and source routers. The source typically offerscontent in more than one multicast group.

Referring to FIG. 2, a diagram of Class D IP addresses is shown. Class DIP addresses are reserved for IP multicasting, and include IP addressesfrom 224.0.0.0 to 239.255.255.255. A range of IP addresses within ClassD is already reserved for PIM-SSM, from 232.0.0.0 to 238.255.255.255(238/8). This range is known as SSM-Range, and is defined by IETF (Meyeret al., “Source-Specific Protocol Independent Multicast in 232/8”, IETFDraft, March 2004). According to one embodiment of the invention a rangeof IP addresses, referred to herein as the multicast Static-Range,within Class D is reserved for static multicasting. Although shown inFIG. 2 as lying below the range of PIM-SSM addresses, the range ofstatic multicasting IP addresses may be a set of any Class D IPaddresses which do not overlap the SSM-Range.

Broadly, in operation any router which receives an IGMP or a PIM messagedetermines the group G defined within the message. If the group G lieswithin the multicast Static-Range, the router does not use the PIMprotocol. Otherwise, the router uses the PIM protocol. In this way, IPmulticasting groups can be defined as static by assigning them an IPaddress lying within the Static-Range. When a host joins a static IPmulticasting group by sending an IGMP JOIN(S,G) message, the host routerbuilds an internal connection between OIF and IIF but does not propagateany PIM-JOIN(S,G) message to upstream routers. When a host leaves astatic multicasting group by sending an IGMP LEAVE(S,G) message, thehost router tears down the internal connections between OIF and IIF butdoes not propagate any PIM-PRUNE(S,G) message to upstream routers. If ahost joins or leaves a dynamic multicasting group, that is, a groupwhose IP address does not lie within the multicast Static-Range, therouters employ either the PIM-SM or PIM-SSM protocol to build and teardown shortest path trees or the RP trees, or to register a multicastsource with an RP in the case of PIM-SM.

Referring to FIG. 3, a flowchart of a method implemented by a router ina communication network according to one embodiment of the invention isshown. At step 30 the router receives either a PIM message or an IGMPmessage. The multicast Static-Range is configured in memory within therouter. The router determines the group G in the message, and at step 32the router determines whether the group G lies within the multicastStatic-Range. If the group G does not lie within the multicastStatic-Range, the router processes the message using conventionaldynamic IP multicast.

If the group G does lie within the multicast Static-Range, the routerdetermines at step 36 whether the message is an IGMP JOIN(S,G) message.If the message is an IGMP JOIN(S,G) message, then at step 38 the routerestablishes a connection between the downstream outgoing interface (OIF)through which the message arrived and the upstream incoming interface(IIF) leading to the source S specified by the message.

If the message is not an IGMP JOIN(S,G) message, then at step 40 therouter determines whether the message is an IGMP LEAVE(S,G) message. Ifthe message is an IGMP LEAVE(S,G) message, then at step 42 the routerremoves the connection between the OIF through which the message arrivedand the IIF leading to the source S specified by the message.

If the message is not an IGMP LEAVE(S,G) message, then at step 42 therouter ignores the message. Any such message will be either a PIMJOIN(S,G) message, a PIM JOIN(*,G) message, a PIM PRUNE(S,G) message, aPIM PRUNE(*,G) message, a PIM REGISTER message, an IGMP JOIN(*, G)message, or an IGMP LEAVE(*, G) message, none of which are to beprocessed for group addresses which lie within the multicastStatic-Range.

Referring to FIG. 4, the example communication network of FIG. 1 isshown following establishment of a Static IP Multicast connectionaccording to one embodiment of the invention. The source 14 offers IPmulticasting for a group G₁ appropriate for Static Multicasting, such asa television channel which will requested and dropped frequently byusers. The group G₁ is a multicast IP address lying within the multicastStatic-Range and is configured on each router. Since there is no dynamicprotocol running in the network for this group (since it lies within themulticast Static-Range), the connection between the downstream OIFs andthe upstream IIFs leading to the source at each router (except the hostrouter 12) must be established by configuration or by an automatedmethod. To establish the multicast tree at the source router 16, thesource router 16 is provided, either manually or by an automated method,with an IGMP JOIN(S,G₁) message at an OIF leading to the downstreamintermediate router 18. As described above with reference to step 38 ofFIG. 3, the source router 16 establishes a connection 50 between the IIFleading to the source 14 and the OIF leading to the intermediate router.Similarly, the intermediate router 18 is provided with an IGMPJOIN(S,G₁) message at an OIF leading to the downstream host router 12,which causes the intermediate router 18 to establish a connection 52between the OIF leading to the host router and the IIF leading to thesource 14. The intermediate router 18 does not propagate a PIMJOIN(S,G₁) message to the upstream source router 16, as the intermediaterouter recognizes that the IP address of the Group G₁ lies within themulticast Static-Range.

Other OIFs at the source router 16 and the intermediate router 18 maysimilarly be provided with IGMP JOIN(S,G₁) messages, if these OIFs leadtowards other routers that may be interested in receiving traffic forthe group G₁. For example, a connection 54 may be established betweenthe same IIF as connection 52 and an OIF leading to a second host router(not shown in FIG. 4).

When the host 10 wishes to receive multicast traffic for the group G₁,the host sends an IGMP JOIN(S,G₁) message to an OIF of the host router12, and the host router 12 establishes a connection 56 between the OIFleading to the host and an IIF leading to the source. The host router 12recognizes that the IP address of the group G₁ lies within the multicastStatic-Range and, contrary to normal PIM-SMM behavior, does not thensend a PIM JOIN(S,G₁) message to the upstream intermediate router 18.This is because the multicast connections on all upstream routerstowards the source 14 have already been established separately ahead oftime for group G₁. Because most of the path leading to the source 14already exists, the host 10 has access to the group G₁ much more quicklythan if Dynamic IP Multicasting was being used and some or all of theshortest path tree had to be established.

Similarly, as described above with reference to FIG. 3, if the host 10leaves the channel, such as by switching to another channel, the hostrouter 12 removes the connection 56 but does not send a PIM PRUNE(S,G₁)message to the intermediate router 18.

If the host 10 wishes to join a different multicast tree defined for agroup G₂, the host 10 sends an IGMP(S, G₂) message to the host router12. If the group G₂ does not lie within the configured Static-Range (asdescribed above with reference to step 32 of FIG. 3), the host router 12processes the IGMP JOIN(S,G₂) message as a dynamic IP multicast. As aresult, the PIM-SSM protocol establishes a connection between the OIFover which the IGMP JOIN(S,G₂) message was received and an IIF leadingtowards the specified source S, and then sends a PIM-JOIN(S,G₂) to theappropriate upstream router. The upstream routers establish internalconnections and propagate PIM-JOIN(S,G₂) messages upstream, inaccordance with usual PIM-SSM behavior, since the IP address of group G₂is not within the multicast Static-Range.

Similarly, if the host 10 wishes to join a multicast tree (*,G₃), thehost sends an IGMP JOIN(*,G₃) message to the host router 12. Being asparse mode multicast, the group G₃ should not lie within theStatic-Range. The host router 12 determines that the IP address of thegroup G₃ does not lie within the multicast Static-Range (as describedabove with reference to step 32 of FIG. 3), and processes the IGMPJOIN(*,G₃) message as a dynamic IP multicast. Internal routerconnections are established and PIM-JOIN(*,G₃) messages are propagatedupstream towards the RP in accordance with usual PIM-SM behavior.

The invention provides flexibility in configuring additional staticmulticasting trees or removing existing multicast trees. In other words,a static multicast tree can be added or removed without interruptingeither static or dynamic networks. Referring to FIG. 5, the examplecommunication network of FIG. 4 is shown following establishment of asecond static multicasting tree according to one embodiment of theinvention. Group G₁ remains configured in the form of connections 50,52, and 54 between respective OIFs and IIFs. A second group G₄, havingan IP address lying within the multicast Static-Range, is offered by asecond source 70 S₂ is configured similarly as is described above forconfiguration of group G₁ with reference to FIG. 4. An IGMP-JOIN(S₂, G₄)message is provided to an OIF on a second source router 72 which leadsto the intermediate router 18. The second source router 72 determinesthat the IP address of the group G₄ lies within the multicastStatic-Range, and establishes a connection between the OIF and the IIFleading to the second source. An IGMP-JOIN(S₂,G₄) message is provided tothe OIF on the intermediate router which leads towards the host router12, and in response thereto the intermediate router 18 establishes aconnection between the OIF and an IIF leading towards the source router72. It can be seen that in establishing the internal connections, theexisting static multicasting group G₁ (originating at source 14) and anydynamic multicasting groups currently in effect are not affected byconfiguration of the second static multicasting group G₄.

The method by which the routers process the PIM control messages,described above with reference to FIG. 3, is preferably carried out inthe form of software instructions within one or more processors, but maymore generally be stored and accessed as instructions in the form of anycombination of software and hardware, including hardware within anintegrated circuit. If in the form of software instructions, thesoftware instructions may be stored on a computer-readable medium.

The invention has been described with reference to a range of Class D IPaddresses reserved for use in static IP multicasting, referred to as themulticast Static-Range. Alternatively, the IP addresses reserved forstatic IP multicasting need not be contiguous. The invention can beeasily understood to be more generally operational with a multicastStatic-Range defined as a set of IP addresses reserved for use in staticIP multicasting being not necessarily contiguous. However, using a setof IP addresses defined as a contiguous range simplifies implementationof the invention as it is easier to determine whether the IP address ofa multicast group identified in a PIM or IGMP message is within acontiguous set specified by upper and lower values.

The embodiments presented are exemplary only and persons skilled in theart would appreciate that variations to the embodiments described abovemay be made without departing from the spirit of the invention. Methodsthat are logically equivalent or similar to the method described abovewith reference to FIG. 3 may be used to implement the methods of theinvention. The scope of the invention is solely defined by the appendedclaims.

1. A method of implementing Internet (IP) Multicasting in acommunication network, the method comprising: defining a multicastStatic-Range, being a set of at least one Class D IP address; reservingeach IP address within the multicast Static-Range for static IPmulticasting; establishing in the network at least one dynamic IPmulticasting group, each dynamic multicasting group having a respectiveIP address lying outside the multicast Static-Range; and establishing inthe network at least one static multicasting group for which PIM-SMMmessaging and PIM-SM messaging is ignored, each static multicastinggroup having a respective IP address lying within the multicastStatic-Range.
 2. A method of supporting coexistence of static IPmulticasting and dynamic IP multicasting at a router in a communicationnetwork, comprising: storing a multicast Static-Range in memory, themulticast Static-Range being a set of at least one Class D IP addressreserved for static IP multicasting; upon receipt of a PIM message or anIGMP message, determining whether the message pertains to a staticmulticasting group; if the message does not pertain to a staticmulticasting group, processing the message in accordance with dynamic IPmulticast; if the message pertains to a static multicasting group andthe message is a PIM message, discarding the message; if the messagepertains to a static multicasting group and the message is an IGMP JOINmessage specifying a source, establishing a connection within the routerand identifiable by the source and the group without forwarding a PIMmessage to an upstream router; and if the message pertains to a staticmulticasting group and the message is an IGMP LEAVE message specifying asource, removing a connection within the router and identifiable by thesource and the group without forwarding a PIM message to an upstreamrouter.
 3. A router for use in a communication network, comprising: amemory for storing a multicast Static-Range, being a set of at least oneClass D IP address, each IP address within the multicast Static-Rangebeing reserved for static multicasting; means for, upon receipt of a PIMmessage, determining whether the message pertains to a static multicastgroup; means for ignoring a received PIM message in the event that themessage pertains to a static multicast group; means for processing areceived PIM message in accordance with PIM protocol in the event thatthe message does not pertain to a static multicast group; means for,upon receipt of an IGMP message, determining whether the messagepertains to a static multicast group; means for, upon receipt of an IGMPJOIN message specifying a source, establishing a connection within therouter without forwarding a corresponding PIM message to an upstreamrouter in the event that the IGMP JOIN message pertains to a staticmulticast group; means for, upon receipt of an IGMP LEAVE messagespecifying a source, removing a connection within the router withoutforwarding a corresponding PIM message to an upstream router in theevent that the IGMP LEAVE message pertains to a static multicast group;and means for processing a received IGMP message in accordance withdynamic IP multicast in the event that the message does not pertain to astatic multicast group.
 4. The router of claim 3, wherein the means fordetermining whether a PIM message pertains to a static multicast groupcomprises determining an IP address of a group specified by the PIMmessage and determining that the message pertains to a static multicastgroup if the IP address of the group lies within the multicastStatic-Range, and wherein the means for determining whether an IGMPmessage pertains to a static multicast group comprises determining an IPaddress of a group specified by the IGMP message and determining thatthe message pertains to a static multicast group if the IP address ofthe group lies within the multicast Static-Range.